Cutting tool for aluminum workpieces having enhanced crater wear resistance

ABSTRACT

An iron-based cutting tool, useful for machining aluminum-based workpieces at surface speeds at or in excess of 1000 sfm, is fabricated by (i) removing surface impurities from the surface of the cutting tool which will be exposed to crater wear during such machining, the removal being by sputtering or ion bombardment using the surface as a target thereby resulting in a cleansed activated surface; and (ii) depositing a single-phase crystal film of sputtered silicon carbide onto the cleansed activated surface. 
     The activation of the surface in step (i) is carried out by causing ionized argon to be accelerated against the surface maintained at about zero bias and the depositing of step (ii) is carried out to cause silicon carbide to be condensed onto the activated surface as a result of being dislodged from a silicon carbide target maintained at about zero bias for at least 300 minutes. The film of silicon carbide has a thickness in a range of about 1-4 microns, and the crystal orientation of such film is amorphous.

This is a Division of application Ser. No. 133,826, filed Dec. 16, 1987now U.S. Pat. No. 4,936,959.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to cutting tools for machining aluminum-basedworkpieces and, more particularly, to the construction, method ofmaking, and method of using a cutting tool that is effective in reducingcrater wear when continuously machining at surface speeds of 1000 sfm ormore.

2. Discussion of the Prior Art

Cutting tools used for continuous machining operations, such as milling,turning, and drilling have utilized various hard wear resistant coatingsystems principally on tool steel or cemented carbides. The specificcoating system has been influenced by two basic considerations: thephysical characteristics of the substrate, and the type of material ofthe workpiece that is to be cut.

Coating systems designed for cutting tools which are to be used to cutiron-based workpieces have been designed with wear resistance as themain characteristic to be improved. To enhance wear resistance ofcemented carbide substrates, very hard particles have been coatedthereon by chemical vapor deposition techniques. Typically, thesecoating systems have been of a multilayer (stratified) character. Thisart is generally depicted in U.S. Pat. Nos. 4,416,670; 4,442,169;4,101,703; 3,955,038; 3,977,061; 4,237,184; and 4,357,382.Unfortunately, chemical vapor deposition techniques are limited tosubstrate materials which can withstand a high temperature duringfabrication, such temperatures being in the range of about 1000° C. Ifsuch chemical vapor deposition techniques were employed on steelsubstrates, the hardened characteristics of the steel would be disruptedor destroyed.

Tool steel used to cut principally iron-based workpieces have also hadcoating systems designed to concentrate accordingly on thecharacteristic of wear resistance. Hard materials, such as carbides andnitrides of titanium, zirconium and hafnium, have been deposited by suchtechniques as ion plating (see U.S. Pat. Nos. 4,169,913 and 4,401,719).Ion plating is desirable because of its tendency to obtain greaterdisorder of the crystal lattice and greater density of the latticedefect to obtain greater wear resistance. Analogously, in U.S. Pat. No.4,341,843, a sputtering technique was employed to deposit a film oftitanium carbide onto a steel substrate for the purpose of achievinghigh wear resistance but not necessarily for use on a cutting tool.

What is missing in the referenced art is a consideration of how suchcoating systems would function for resisting crater wear. Crater wear isa combination phenomenon resulting in high temperature tearing andenergy impact at high surface speeds of cutting, as well as hightemperature oxidation. Crater wear is particularly troublesome withrespect to cutting aluminum workpieces. Although wear resistance is ofsome importance, it is not as important as crater wear resistance. It iswith this specific set of conditions that this invention is concerned,the use of iron-based substrates to machine aluminum-based workpieces athigh surface speeds. Thus, the relevant prior art would be coatingsystems designed for these specific conditions. Applicants are unawareof any prior art which discloses coating systems designed specificallyfor such applications.

SUMMARY OF THE INVENTION

The invention, in one aspect, is a method of making an iron-basedcutting tool useful for machining aluminum-based workpieces at surfacespeeds at or in excess of 1000 surface feet per minute (sfm). Suchmethod comprises (i) removing surface impurities from the surface of thecutting tool which will be exposed to crater wear during the machining,the removal being by sputtering or ion bombardment using the surface asa target thereby resulting in a cleansed activated surface; and (ii)depositing a single-phase crystal film of sputtered silicon carbide ontothe cleansed activated surface.

The activation of the surface in step (i) is preferably carried out bycausing ionized argon to be accelerated against the surface maintainedat about zero bias for a period of about 2-4 minutes; preferably, thedepositing of step (ii) is carried out to cause silicon carbide to becondensed onto the activated surface as a result of being sputtered froma silicon carbide target for at least 300 minutes. Advantageously, thefilm of silicon carbide has a thickness in a range of about 1-4 microns,and the crystal orientation of such film is amorphous.

The invention, in another aspect, is a cutting tool for machiningaluminum workpieces at surface speeds in excess of 1000 sfm, the toolhaving enhanced crater wear resistance. The tool comprises (a) apreshaped base of steel having a cutting edge defined by the juncture offlank and rake surfaces, and (b) a single-phase crystal film ofsputtered silicon carbide coated onto at least the rake surface.Advantageously, the cutting tool has a crater wear resistancerepresented by a K value of less than 0.08 when measured at a distanceof 0.03-0.045 inches from the cutting edge and viewed at an angle ofzero or 30° from the flank cutting edge.

Still another aspect of this invention is a new method of use, the usebeing for machining aluminum-based workpieces at high surface speedsexceeding 1000 sfm. The method of use comprises moving an iron-basedcutting tool against an aluminum-based workpiece to continuously shearoff a shaving of said workpiece, the cutting tool having a rake facecontacted by the shaving, which face is coated with a single-phasecrystal film of sputtered silicon carbide.

SUMMARY OF THE DRAWINGS

The FIGURE is a schematic view of a cutting tool of this invention forcarrying out machining against an aluminum workpiece.

DETAILED DESCRIPTION AND BEST MODE

High speed steel cutting tool inserts coated with a thin single-phaselayer of silicon carbide by a dual ion beam sputtering technique show amarked improvement in resistance to crater wear when machiningaluminum-based workpieces.

High speed tool steels typically have designations of the M or T type.Those with an M designation contain tungsten, molybdenum, chromium andvanadium as alloying ingredient and sometimes cobalt. The tungsten willbe in the range of 1.5-20% by weight, the chromium in the range of3.75-4.50% by weight, and vanadium in the range of 1-5.0% by weight.Molybdenum, when present, will preferably be in the range of 3.75-9.5%,and cobalt, when present, in the range of 5-12%. Carbon, for high speedtool steels, is in the range of 0.75-1.25% by weight. However, thisinvention is believed to be applicable to all iron-based cutting toolsubstrates; iron based is used herein to mean metal cutting toolsubstrates which have an iron content in excess of 50% by weight.Similarly, aluminum-based is used herein to mean a workpiece havingaluminum content in excess of 50% by weight.

Tool steels require special heat treatment in order that their uniqueproperties may be fully realized. For example, a special heat treatmentof heating to a high temperature (2150°-2400° F. or 1176°-1315° C.) toobtain solution of substantial percentage of the alloy carbides,quenching to room temperature, at which stage a considerable amount ofthe austenite is retained tempering at 1000°-1150° F. or 538°-621° C.,and again cooling to room temperature. During tempering, alloy carbidesare precipitated, resulting in marked secondary hardening and areduction of alloy content in the retained austenite which thentransforms to martensite on cooling to room temperature and results in astill greater hardness increase. It is often desirable to temper asecond time to change the martensite formed on cooling from the originaltempering. This very time consuming and expensive heat treatment processshould not be dissipated as a result of the coating technique.

The cutting tools of interest to this invention are of the type employedto carry out machining operations of a shearing type; this requires thatthe cutting tool have a configuration having a flank and a rake surfacewhich together define a cutting edge. As the cutting tool is movedagainst the aluminum workpiece, the cutting edge shears off a shavingwhich contacts the rake surface as it is peeled therefrom. When suchmachining operation is carried out at speeds of 1000 sfm and in excessthereof, the temperature condition of the shaving and the work tool areat extremely high levels, predictably in the range of 450°-500° C. foraluminum and 500°-1000° C. for steel.

To deposit the coating system of this invention onto the rake face, thecutting tool substrate is placed obliquely above a silicon carbidetarget on an electrically conducting block. The block may be placed on arotatable table that is grounded and supports the block. The substratesmay be about six centimeters above the target. Together, the substrate,block and table may be positioned to face either a sputter etch gun orthe SiC target. Target cooling may be supplied through a convenienttube. Zero voltage is applied directly to the cutting tool substratethrough the support for both cleaning by sputter etching and for filmdeposition. Two sources of ion beams are employed, first a beam of argonions for cleaning purposes, and another beam to promote sputtering ofthe SiC target.

Cleaning of the cutting tool is accomplished by sputter etching with a1000 volt, 95 mA beam for about 2-4 minutes. The cleaning and coatingoperations are performed in a vacuum chamber which houses the target aswell as the grounded table. A regulated flow of high purity argon isselectively introduced into the ion source through a nozzle and directedby the etch gun to produce an ion beam aimed at the tool surface. Asecond deposition gun is directed at the SiC target. The substrate orsubstrates are positioned to face the sputter etch gun. Both guns arestarted. Upon completion of about one minute of etching, the substrateposition is graduated towards the film coating position. Upon completionof three minutes of sputter etching, the substrates are positioned tothe film coating position. The sputter etch gun is then stopped andcoating time is calculated to begin; coating deposition is carried outfor at least 30 minutes to obtain a coating thickness of 1-4 microns.

EXAMPLES

To corroborate the scope of this invention, several high speed steelcutting tool inserts were coated according to various parameters of thisinvention, using a silicon carbide compound target that was sputteredwith a beam of argon ions accelerated to 1500 volts at 70 mA in a narrowbeam ion source. The high speed tool steel was of the M-2 type, that is,it contained 0.85% carbon, 6% tungsten, 5% molybdenum, 4% chromium, and2% vanadium, with no cobalt present.

The resulting silicon carbide vapor condensed on the steel inserts,which had been sputter cleansed using an argon ion beam from a first ionsource prior to and during the initial stages of deposition. This yieldsan adherent coating of silicon carbide approximately two microns thick(0.0001 inches) on the rake face of the tool insert with lesser amountson the flank and end faces of the tool insert.

Such coated high speed tool steel inserts were deployed in turning testsat 1000 and 2000 sfcm cutting speeds against a 333 aluminum alloyworkpiece in the sodium modified as well as the unmodified conditions.The cutting depth was 0.040 inches, the feed was 0.007 inches perrevolution, and a water soluble oil emulsion coolant was used in all ofthe tests. The tool insert was configured in accordance with styleSPG-422. Both uncoated high speed cutting tool inserts were employed aswell as those coated with silicon carbide as above indicated, and in twoinstances titanium carbide and titanium nitride were employed assputtered coatings for comparative purposes. Results of these machiningtests are shown in Table I. "K" values shown for crater depth weremeasured using a profile tracing instrument called a profilometer at anangle of 30° from the flank cutting edge and at distances of 0.030inches and 0.045 inches from the end cutting edge. A "K" value isdefined by the equation K=K_(t) /K_(m) where K_(t) represents craterdepth at its midpoint and K_(m) represents the crater width.

As shown in Table I, the crater depth after machining with the sputteredsilicon carbide coated insert was in all cases significantly lower thanthat obtained using an uncoated tool, or a tool insert coated withtitanium carbide or titanium nitride. In fact, in two of the casesshown, no-measurable crater was present at all. Since cratering, andespecially crater "breakthrough" are often the tool life end-points ofhigh speed steel tools, the observed improvement in resistance tocratering represents a significant achievement toward lengthening theuseful life of such steel tools when machining aluminum-basedworkpieces.

While particular embodiments of the invention have been illustrated anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention, and it is intended to cover in the appended claims all suchmodifications and equivalents as fall within the true spirit and scopeof the invention.

                  TABLE I                                                         ______________________________________                                        CRATER WEAR ON SiC - COATED AND UNCOATED                                      M-2 HIGH SPEED STEEL INSERTS                                                  (Workpiece: 333 Al Alloy, Coolant: 5% 589B,                                   Feed: .007 ipr, Style: SPG-422, Depth: .040 in.)                                        Speed   K (0°)                                                                          K (30°)                                                                       K (30°)                                                                       Na                                   Tool Condition                                                                          (sfpm)  (.030 in.)                                                                             (.030 in.)                                                                           (.045 in.)                                                                           Modif                                ______________________________________                                        Uncoated  1000    .0857    .078    .0704 Yes                                  SiC Coated                                                                              1000    0        0      0      Yes                                  Uncoated  1000    .1458     .1075  .1387 No.                                  SiC Coated                                                                              1000    .0493    .074   .038   No                                   Uncoated  2000    .1512    .212   .171   No                                   SiC Coated                                                                              2000    0        0      0      No                                   ______________________________________                                    

We claim:
 1. A cutting tool for machining aluminum-based workpieces atsurface speeds equal to or in excess of 1000 sfm and having enhancedcrater resistance, said tool having flank and rake surfaces,comprising:(a) a preshaped base of steel having a cutting edge definedby the juncture of said flank and rake surfaces; (b) a simple-phasecrystal film of sputtered silicon carbide coated directly onto at leastsaid rake surface, said cutting tool being characterized by a reductionin crater wear resistance of at least 0.05 in K value when (i) measuredat a distance of 0.03 inches from the cutting edge, (ii) viewed at anangle of zero degrees from the flank cutting edge, (iii) compared to acutting tool devoid of said sputtered silicon carbide film and (iv)cutting is carried out under the same conditions and parameters.